Coding And Noncoding Sequences
The amino acid sequence of proteins is directed by the information found in genes, which in turn are made up of DNA. Genes that have different DNA sequences are said to be polymorphic. These different gene forms are called alleles, exemplified by the alleles that control eye color. When alleles result in differences in the amino acid sequence of a protein, the proteins encoded by alleles are called isoforms. The position of the gene on a chromosome is its locus (plural, loci). More generally, a locus refers to any position on a chromosome, whether or not a gene is located there.
Polymorphisms arise through mutation. The mutation may be due to a change from one type of nucleotide to another, an insertion or deletion (collectively known as indels), or a rearrangement of nucleotides. Once formed, a polymorphism can be inherited like any other DNA sequence, allowing its inheritance to be tracked from parent to child.
Polymorphisms are also found outside of genes, in the vast quantity of DNA that does not code for protein. Indeed, regions of DNA that do not code for proteins tend to have more polymorphisms. This is because changes in DNA sequences that encode proteins may have a harmful effect on the individual that carries it. Polymorphisms that do not have any effect on the organism are said to be selectively neutral since they do not affect its ability to survive and reproduce.